Pollination Biology of Montane Plants: Relationship Between Rate of Nectar Production and Standing Crop

Abstract

Rates of floral nectar production and volumes of standing crops of floral nectar were measured for eight species of Colorado montane plants. Although significant positive correlations exist between the two variables, the magnitudes of the coefficients are not very great. This surprising result may be due to the large variability in rate of nectar production within each species. The patterns suggest that pollinators, by responding to standing crop volumes, may not exert very strong selective pressure on rate of floral nectar production. All species examined had similarly variable patterns of standing crop indicating that pollinators encounter a wide range of reward variability while collecting nectar, regardless of which plant species is being visited. INTRODUCTION It has become increasingly obvious in recent years that successful plant reproduction occurs not only as a function of the frequency of pollinator visitation but of subtler aspects of pollinator behavior as well (e.g, Pyke, 1981; Waser, 1983; Zimmerman, 1988). Not all combinations of matings, for example, yield equal results. Price and Waser (1979) and Waser and Price (1983) have found that optimal outcrossing distances exist for both Delphinium nelsonii and Ipomopsis aggregata; pollen moving this distance before landing on a stigma has a higher probability of producing more, high-quality seeds than does pollen arriving from either a shorter or longer distance. Also, a growing body of literature suggests that pollinator flight distances, and thus, ultimately, the distances pollen grains are carried, are significantly influenced by the amount of nectar reward received from flowers by pollinators (e.g., Pyke, 1978a; Heinrich, 1979; Zimmerman, 1982; Galen and Plowright, 1985; Zimmerman and Cook, 1985). Therefore the possibility exists that pollinators might provide selective pressure on plants to produce optimal quantities of floral nectar (Zimmerman, 1988). The concept that rate of floral nectar production by plants might be under selective pressure from pollinators is certainly not novel (Pyke, 1981; Pyke and Waser, 1981; Pleasants, 1983; Pleasants and Chaplin, 1983). Nectar production in a number of species has been found to have high heritability (Pedersen, 1953; Hawkins, 1971; Teuber and Barnes, 1978, 1979; Teuber et al., 1980, 1983) and to respond quickly to artificial selection (Hawkins, 1971). Although both of these characteristics are necessary if pollinators are to provide the selective force necessary to influence rate of nectar production, a third condition must also be met. Selection will operate only if a significant, positive relationship exists between rate of nectar production and standing crop of nectar. From the perspective of pollinators, standing crop is the more important of the two values because it, more directly, reflects the nature of the reward received. Standing crops, however, are determined both by the rate of nectar production and by the foraging behavior of pollinators. Surprisingly, not a single study has examined the relationship between rate of floral nectar production and the volume of standing crop of nectar found in flowers. It is certainly reasonable to expect that a significant positive relationship between nectar production and standing crop will exist. What is important from an evolutionary standpoint, however, is the strength of that relationship. Selection pressure on rate of nectar production cannot be very great when the two variables are only weakly linked. A weak correlation can arise in at least two manners. First, if high per flower visitation